1. Field of the Invention
This invention relates to a method of removing sulfur and nitrogen oxides by a dry process and more particularly, it is concerned with a method of removing sulfur and nitrogen oxides contained in a waste gas by adding ammonia to the waste gas, introducing the waste gas into a packed bed or moving bed (which will hereinafter be referred to as "packed bed" simply) of a carbonaceous adsorbent and preventing unreacted ammonia from flowing out.
2. Description of the Prior Art
Of late, as a method of removing sulfur oxides and nitrogen oxides in a waste gas, it has been of keen interest in the art to add ammonia to a waste gas and to pass the waste gas through a packed bed of carbonaceous adsorbent, because this is characterized in not requiring water in principle, unlike the wet process desulfurization method represented by the lime gypsum method, in not needing to reheat the waste gas after processing and the addition of ammonia removes the nitrogen oxides from the waste gas. This method has the feature that if the reaction temperature is low, the desulfurization reaction is advantageous, while if the reaction temperature is high, the denitrification reaction is advantageous. On the other hand, the temperature of a waste gas from a boiler plant is generally about 150.degree. C., but there has lately been a tendency to lower the temperature of a waste gas from a boiler for the purpose of raising the thermal efficiency of the boiler, i.e. to save energy. This is a condition suitable for the desulfurization reaction, but more unsuitable for the denitrification reaction in an apparatus for removing sulfur and nitrogen oxides using a carbonaceous adsorbent.
In order to obtain a high removal efficiency of nitrogen oxides where the temperature of a waste gas is relatively low, e.g. 110.degree.-180.degree. C., it has hitherto been proposed to remove previously the bulk of the sulfur oxides from the waste gas and to add ammonia to the waste gas to effect a denitrification reaction (Japanese patent application OPI No. 129131/1980). However, it is necessary to increase the amount of ammonia to obtain a high denitrification efficiency by this method, but the reaction temperature is not sufficiently high for the denitrification reaction and a large amount of unreacted ammonia flows out, thus bringing about the danger of secondary pollution from an apparatus used for preventing environmental pollution. On the contrary, if the additive amount of ammonia is decreased so that unreacted ammonia is not allowed to flow out, the denitrification efficiency is lowered. Accordingly, it is impossible to practive, on a commercial scale, a method of removing nitrogen oxides using a carbonaceous adsorbent at the temperature of a boiler waste gas.